Not sure what he'll prove that hasn't already been done with weather balloons

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There has never been any "proof" of horizon level from any weather balloon because that's impossible.
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There never has been and there never will be.
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There are no telescope systems used inside the earth's atmosphere unless they're on solid ground.
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All telescope systems above solid ground have to be outside the earth's atmosphere, in space.
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In outer space where there is no contact with earth's air layer the telescope can be kept motionless.
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Weather balloons are incapable of keeping steady because the air movement all around them makes them move too.
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There is no instrument system that can keep a telescope motionless mounted to a weather balloon.
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Therefore weather balloons cannot support the necessary instruments to view the horizon using a level scope.
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When someone claims to have obtained a level telephoto of the horizon from a weather balloon you know it's fake.
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You have to have solid ground to support your instruments, such as a mountain top.
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Anyone with experience using telescopes knows this.
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It can't be done from the air, from a blimp, from an aircraft or from a "steam powered rocket."
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It can't be done while hang-gliding or free-falling like in sky diving.
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You need solid ground, and you have to take your measurements while there is no active earthquake happening.
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There are more earthquakes happening than you might think.
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Flat horizon zoomed in tennis ball.